1. Field of the Invention
The present invention primarily relates to an image forming apparatus utilizing an electrophotographic method. The image forming apparatus is, for example, an electrophotographic copying machine, an electrophotographic printer (such as an LED printer and a laser beam printer), or an electrophotographic facsimile machine.
2. Description of the Related Art
Hitherto, in image forming apparatuses such as an electrophotographic copying machine and printer, contact charging is performed by discharge from a charging member to a charged member (image bearing member). Therefore, the charging to the image bearing member is started by applying a voltage of not lower than a certain threshold value to the charging member. For example, when a charging roller (charging member) is pressed and contacted with an OPC photosensitive member (image bearing member) having a predetermined thickness, the surface potential of the photosensitive member starts to rise by applying a voltage of not lower than the discharge start voltage. Thereafter, the surface potential of the photosensitive member is linearly increased at a gradient of 1 with respect to the applied voltage. In the following description, the threshold voltage at which the discharge is started is defined as the discharge start voltage Vth.
In order to obtain a desired surface potential Vd of the photosensitive member, therefore, a voltage of Vd+Vth is required to be applied to the charging roller.
That principle can be explained as follows. The charging roller, an air layer formed by a minute gap between the charging roller and the photosensitive member, and the photosensitive member can be expressed by an electric equivalent circuit.
The impedance of the charging roller is not handled here because it is so small as to be negligible compared to the impedance of each of the photosensitive member and the air layer. In other words, a charging mechanism can be expressed just by using two capacitors C1 and C2 (C1 represents the electrostatic capacitance of the photosensitive member and C2 represents the electrostatic capacitance of the air layer).
When a DC voltage V is applied to the equivalent circuit, the applied voltage is distributed to the capacitors in proportion to their impedances. Thus, the voltage applied to the air layer A is given by:Vair=C1/(C1+C2)  (1)
The air layer A has a dielectric breakdown voltage according to the Paschen's law. Assuming the thickness of the air layer A to be d [μm], therefore, discharge occurs and charging is started when Vair exceeds:312+6.2d [V]  (2)The voltage at which the discharge first occurs is given when a quadratic equation with a variable d has a multiple root on condition that the formula (1) and the formula (2) are equal to each other (C2 is also a function of d). V satisfying the above assumption corresponds to the discharge start voltage Vth. A thus-obtained theoretical value of the discharge start voltage Vth shows a very good match with an experimental value.
Further, in a constant-voltage control circuit, Vth is not changed regardless of change in the process speed (i.e., the peripheral speed of the photosensitive member). Such a property can be explained based on the following relation formulae (3) and (4);I=∈·∈0·L·Vp·Vd/d  (3)(I: charging current, ∈: dielectric constant of the photosensitive member, ∈0: dielectric constant in vacuum, L: effective charging width, Vp: process speed, Vd: surface potential of the photosensitive member, and d: film thickness of the photosensitive member), andV=((d/∈·L·Vp)+R))I−Vth  (4)(V: applied voltage to the charging roller, d: film thickness of the photosensitive member, ∈: dielectric constant of the photosensitive member, L: effective charging width, Vp: process speed, R: resistance value of the charging roller, I: charging current, and Vth: discharge start voltage).
In the constant-voltage control circuit, as seen from the formula (3), the process speed and the charging current are proportional to each other. Hence, as the process speed increases, the charging current is also increased proportionally. Further, since the relationship among the applied voltage, the surface potential of the photosensitive member, and Vth is expressed by the formula (4), the process speed and the charging current are canceled to each other, thus resulting in no changes in V and Vth.
Accordingly, in the constant-voltage control circuit, Vth is not changed regardless of change in the process speed (see FIG. 2). Moreover, at voltages of not lower than Vth, it is understood that the applied voltage and the surface potential of the photosensitive member is in a linear relation with a gradient of 1.
However, Vth is changed (see FIGS. 3 and 4) if the electrostatic capacitance C1 of the charged member is changed due to abrasion with the use for a long time (i.e., change in film thickness of the surface member), or if the electrostatic capacitance of the charging roller is changed due to environmental changes.
If the electrostatic capacitance C1 of the charged member is changed due to, e.g., abrasion with the use for a long time (i.e., change in film thickness of the surface layer of the photosensitive member), the discharge start voltage Vth is changed and the charged potential of the charged member is also changed with the change of Vth. In the case of an image forming apparatus, if the electrostatic capacitance C1 is changed due to, e.g., abrasion of the surface of the photosensitive member which is caused with the continued use of an image bearing member (photosensitive member) serving as the charged member, Vth is changed. The change of Vth may shift the charged potential from an initially set desired value and may disturb an image.
Stated another way, when charging is performed at a constant voltage based on the above-described contact charging principle, Vth is changed if the photosensitive member is abraded and the electrostatic capacitance C1 of the photosensitive member is changed. More specifically, because of the relationship ofC1=∈S/t (∈: dielectric constant of the photosensitive member, S: discharge area (constant), and t: thickness of the photosensitive member),C1 is increased if the thickness of the photosensitive member is reduced.
On the other hand, the impedance of the photosensitive member is inversely proportional to C1. Therefore, if the thickness of the photosensitive member is reduced (C1 is increased), the voltage applied to the photosensitive member is reduced and the voltage applied to the air layer is increased on the contrary. This means that, even with the application of the same voltage V, the discharge is more apt to occur and a value of Vth is necessarily reduced after the use for a long time.
Further, in a low-temperature and low-moisture environment (environment at 15° C. and 10% RH or below in the present invention, hereinafter referred to as an L/L environment), the electrostatic capacitance of the charging roller is changed although it is negligible in a normal-temperature and normal-moisture environment (N/N environment). Such a change increases the impedance of the charging roller. Therefore, an extra voltage is required to start the discharge and Vth is increased correspondingly.
In an image forming apparatus utilizing the contact charging, when the apparatus is controlled as usual by employing a constant voltage of (Vd+Vth), which is usually obtained in an initial state of the environment, while ignoring the influence of sheet passage in the use and the influence of the environment, Vth is reduced and Vd is increased if the film thickness of the surface layer of the photosensitive member is decreased with the use. Also, in the L/L environment, because Vth is increased, Vd is reduced. Anyway, there arises a problem that an image is changed. To cope with such a problem, voltage control using an expensive sensor, e.g., an environment sensor, is required.
As the related art addressing the above-mentioned problem, Japanese Patent No. 3214120 proposes a known method of suppressing a variation in potential of an image bearing member, which is caused by environmental variations and a variation in film thickness of the image bearing member. With the proposed known method, a DC voltage is applied to a charging member and a value of the applied voltage is detected at the time when a small current of not larger than 0.5 μA flows between the charging member and the image bearing member. The voltage detected at that time is regarded as a value almost close to the discharge start voltage. By performing voltage control using a value that is obtained by adding a predetermined voltage to the detected voltage, the potential of the image bearing member is held constant regardless of the environmental variations and the variation in film thickness of the image bearing member.